The present invention relates to polarization mode compensators in optical communications systems.
Within each link in a fiber optic communications system, different polarizations of an optical signal traversing the link may travel at different speeds. This effect is called polarization mode dispersion (PMD). The timing difference between two principal states of polarization is called the differential group delay (DGD). Polarization mode dispersion causes the broadening of optical pulses, which is detrimental to signal quality in high bit rate transmission. Making the matter more complex, DGD generally varies as a function of time in a given link.
Conventionally, a fixed DGD compensator is placed at the end of the link to reduce the PMD effect. However, if conditions on the link change, the fixed DGD compensator is not able to change the amount of compensation without being replaced with a different DGD compensator. This method of DGD compensation does not allow for real time DGD compensation and is thus inflexible.
Accordingly, there exists a need for an improved DGD compensator. The improved compensator should be tunable, thus producing variable DGD compensation. The improved compensator should allow for real time DGD compensation. The present invention addresses such a need.
An improved digital differential group delay (DGD) controller is capable of producing variable DGD used to compensate for DGD in a fiber optic link in real time. The DGD controller comprises one or more sets of a polarization modulator optically coupled with a fixed delay component, such as a birefringent plate. Each set provides an amount of DGD compensation. By controlling the number of stages in the controller, the amount of DGD compensation provided by each stage, and which stages are placed in the ON state, the total amount of DGD compensation provided by the digital DGD controller can be varied.